Add script to clean materialized plugin files

Adds eng/clean-materialized-plugins.mjs which removes agents/, commands/,
and skills/ subdirectories from plugin folders. These are generated by CI
via eng/materialize-plugins.mjs during publish and should not be tracked on
the staged branch.

Runnable via: npm run plugin:clean

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

package.json

@@ -15,6 +15,7 @@
     "plugin:create": "node ./eng/create-plugin.mjs",
     "skill:validate": "node ./eng/validate-skills.mjs",
     "skill:create": "node ./eng/create-skill.mjs",
+    "plugin:clean": "node ./eng/clean-materialized-plugins.mjs",
     "plugin:generate-marketplace": "node ./eng/generate-marketplace.mjs",
     "website:data": "node ./eng/generate-website-data.mjs",
     "website:dev": "npm run website:data && npm run --prefix website dev",